Activated T cells play a pivotal role in allograft rejection. Upon activation T cells express gp39 a member of the TNF cytokine superfamily. CD40, the receptor for gp39 is expressed on a wide variety of cells, including dendritic cells, B cells, macrophages, endothelial cells (EC), and T cells. Evidence for the crucial role of gp39 in humoral immunity came from the recognition that the hyperIgM syndrome results from a defect in the gp39 gene. In addition, it has become increasingly apparent that CD4O also plays an important role in the regulation of macrophage, dendritic cell, EC and T cell function. Our central hypothesis is that the CD4O pathway plays a crucial role in the transition between the afferent and efferent phases of allograft rejection. In particular, CD40/gp39 signals are necessary for the delivery of cognate T cell help for effector cell activation and for T cell clonal expansion. We will address this hypothesis in the following specific aims: i) To define the kinetics and distribution of CD4O and gp39 expression during. allograft rejection. 2) To study the role of CD40/gp39 interactions in the T-dependent macrophage activation. Our hypothesis being that CD4O signals delivered during cognate interactions with T cells activate macrophage to express effector molecules and to exhibit effector functions. 3) To study the role of CD4O/gp39 interactions in the regulation of EC activation and leukocyte traffic during allograft rejection. We will test the hypothesis that CD4O/gp39 interactions play a critical role in the regulation of lymphocyte recruitment into allografts. 4) To study the role of CD40/gp39 interactions in the regulation of T cell clonal expansion. We hypothesize that CD4O signals antagonize pro-apoptotic signals delivered via the fas pathway. Deprivation of CD4O signals during T cell responses allows fas signals to predominate, leading to "premature" activation-induced apoptosis, aborting clonal expansion. 5) To explore the mechanisms by which CTLA4-Ig and anti-gp39 synergize to inhibit allo-immune responses. Our hypothesis is that simultaneous blockade of CD28 and CD4O signals during antigen-challenge further shifts the balance toward accelerated fas-mediated activation-induced apoptosis. The CD4O pathway is distinct from other pathways previously targeted to inhibit allograft rejection. Unlike other agents anti-gp39 mAbs are not potent inhibitors of T cell activation. Rather CD4O appears to be a pivotal molecule in the transition from the afferent to the efferent phase of immune responses. As a regulator of many facets of T-dependent immune responses including T cell help for B cell, macrophage and EC activation, as well as T cell clonal expansion, further understanding the biology of the CD4O pathway promises to yield a new class of agents to therapeutically manipulate immune responses.